1. Field of the Invention
This invention relates generally to the field of metal halides and, more particularly, to a process for the preparation of thorium fluoride which is highly transmissive to 10.6 micrometer radiation.
2. Description of the Prior Art
In order to be effective, a material which is used to form an optical component, such as a laser window or optical fiber, has to be transparent to the selected wavelength of radiation it must transmit. In the particular case where a carbon dioxide (CO.sub.2) laser is used as the light source, as is frequently done at the present time, the associated optical components must be transmissive to 10.6 micrometer radiation. However, it is known that polyatomic impurities degrade the optical transparency of the host material in the near infrared radiation range (i.e. 0.75 to 15.0 micrometers). This effect has been studied in detail with regard to alkali halide crystals, as discussed by C. J. Duthler in the publication entitled "Extrinsic absorption in 10.6-.mu.m-laser-window materials due to molecular-ion impurities," in the Journal of Applied Physics. Vol. 45, 1974, pages 2668 to 2671. In particular, water and water vapor pose a difficult problem because these species are ubiquitous impurities that are often uncontrolled in various phases of materials preparation and processing. The impurities derived from water, namely, hydroxyl ions and hydrogen ions, degrade the near infrared transmission of alkali metal halides, as discussed in U.S. Pat. No. 3,932,597, assigned to the present assignee, and of alkaline earth metal halides, as discussed in U.S. Pat. No. 3,935,302, assigned to the present asignee.
Thorium fluoride (ThF.sub.4) has recently been found to be useful for, among other things, forming thin film reflectors and anti-reflectors for use in high power carbon dioxide laser systems. When used as a reflector, the ThF.sub.4 is provided as a thin film on a suitable substrate which is external to the laser resonator cavity, and the ThF.sub.4 film serves to deflect the laser beam in a predetermined direction toward the target. It is, of course, desirable to deflect the laser beam efficiently in order to prevent losses in the laser beam intensity. When used as an anti-reflector, the ThF.sub.4 may be coated on the surface of a window through which the laser beam must pass, such as a window through which the laser beam leaves the laser resonator cavity or a window in a chamber holding a sample, through which the laser beam passes to strike the sample. When used as an anti-reflector, the ThF.sub.4 provides a refractive index at the window surface such that reflection of the laser beam is minimized and transmission of the laser beam through the window is maximized. However, in order to be suitable for such purposes, the ThF.sub.4 must have a high transmission and low absorption for the 10.6 micrometer radiation from the carbon dioxide laser so that the film will not become heated sufficiently to cause destruction thereof, as discussed in greater detail in relation to Example 2 herein. It should be noted that the term "approximately 10.6 micrometers" is used herein to designate the radiation emitted from a carbon dioxide laser.
One problem with ThF.sub.4 which has been studied in the past is the hygroscopic nature of ThF.sub.4 powder, which is used as the starting material to deposit a thin film of ThF.sub.4. While the uptake of water by the ThF.sub.4 powder does not lead to hydration or hydrolysis at ordinary temperatures, the ThF.sub.4 crystal hydrolyzes at approximately 350.degree. C. The latter behavior makes the growth of ThF.sub.4 crystals or films from a melt at 1120.degree. C. sensitive to the presence of water in the vapor phase. One method of overcoming this problem is described in the publication entitled "Preparation and Crystal Growth of ThF.sub.4," by R. C. Pastor and K. Arita, in the Materials Research Bulletin, Vol. 9, 1974, pages 579 to 584, in which a combination of wet and dry conversions was used. First, thorium oxide (ThO.sub.2) powder suspended in water was reacted with a solution of hydrofluoric acid (HF) and the product, in the form of a residue, was determined to be 0.914 ThF.sub.4 and 0.086 ThF.sub.4.4H.sub.2 O. The residue was then treated with dry HF overnight at 500.degree. C. and then for one hour at 700.degree. C. While the ThF.sub.4 powder so formed was shown to be significantly improved over some commercially available ThF.sub.4 powders, which were either 19 percent hydrolyzed or incompletely converted to the extent that 3.5 mole percent of ThO.sub.2 remained unconverted, the former material still has some residual degree of hydrolysis or incomplete conversion. However, the ThF.sub.4 powder of Pastor and Arita was found to be a suitable starting charge for crystal growth of ThF.sub.4 having certain improved properties. Nevertheless, even this improved ThF.sub.4 was later found to be unsuitable for forming films with very low optical absorption at 10.6 micrometers.
It is the alleviation of this prior art problem of the optical absorption of ThF.sub.4 films at approximately 10.6 micrometers to which the present invention is directed.